Liquid crystals can exist in various phases. In essence there are three different classes of liquid crystalline material, each possessing a characteristic molecular arrangement. These classes are nematic, cholesteric, (chiral nematic) and smectic. A wide range of smectic phases exists, for example smectic A and smectic C. Some liquid crystal materials possess a number of liquid crystal phases on varying the temperature, others have just one phase. For example, a liquid crystal material may show the following phases on being cooled from the isotropic phase: isotropic-nematic-smectic A-smectic C-solid. If a material is described as being smectic A then it means that the material possesses a smectic A phase over a useful working temperature range.
Ferroelectric smectic liquid crystal materials, which can be produced by mixing an achiral host and a chiral dopant, use the ferroelectric properties of the tilted chiral smectic C, F, G, H, I, J and K phases. The chiral smectic C phase is denoted S.sub.C * with the asterisk denoting chirality. The S.sub.C phase is generally considered to be the most useful as it is the least viscous. Ferroelectric smectic liquid crystal materials should ideally possess the following characteristics: low viscosity, controllable spontaneous polarisation (Ps) and an S.sub.C phase that persists over a broad temperature range, which should include ambient temperature and exhibits chemical and photochemical stability. Materials which possess these characteristics offer the prospect of very fast switching liquid crystal containing devices. Some applications of ferroelectric liquid crystals are described by J. S. Patel and J. W. Goodby in Opt. Eng., 1987, 26, 273.
In ferroelectric liquid crystal devices the molecules switch between different alignment directions depending on the polarity of an applied electric field. These devices can be arranged to exhibit bistability where the molecules tend to remain in one of two states until switched to the other switched state. Such devices are termed surface stabilised ferroelectric devices, eg as described in U.S. Pat. No. 5,061,047 and U.S. Pat. No. 4,367,924 and U.S. Pat. No. 4,563,059. This bistability allows the multiplex addressing of quite large and complex devices.
It is well known in the field of ferroelectric liquid crystal device technology that in order to achieve the highest performance from devices, it is important to use mixtures of compounds which give materials possessing the most suitable ferroelectric smectic characteristics for particular types of device. The thermal and physical properties of the device mixture may be finely tuned by adjusting the concentrations and nature of the components in the mixture.
Cholesteric or chiral nematic liquid crystals possess a twisted helical structure which is capable of responding to a temperature change through a change in the helical pitch length. Therefore as the temperature is changed then the wavelength of the light reflected from the planar cholesteric structure will change and if the reflected light covers the visible range then distinct changes in colour occur as the temperature varies. This means that there are many possible applications including the areas of thermography and thermooptics.
The cholesteric mesophase differs from the nematic phase in that in the cholesteric phase the director is not constant in space but undergoes a helical distortion. The pitchlength for the helix is a measure of the distance for the director to turn through 360.degree..
By definition a cholesteric material is a material which contains a chiral centre. Cholesteric materials may also be used in electrooptical displays as dopants, for example in twisted nematic displays where they may be used to remove reverse twist defects. They may also be used in cholesteric to nematic dyed phase change displays where they may be used to enhance contrast by preventing wave-guiding.
Thermochromic applications of cholesteric liquid crystal materials usually use thin-film preparations of the cholesterogen which are then viewed against a black background. These temperature sensing devices may be placed into a number of applications involving thermometry, medical thermography, non-destructive testing, radiation sensing and for decorative purposes. Examples of these may be found in D. G. McDonnell in Thermotropic Liquid Crystals, Critical Reports on Applied Chemistry, Vol. 22, edited by G. W. Gray, 1987 pp 120-44; this reference also contains a general description of thermochromic cholesteric liquid crystals.
Generally, commercial thermochromic applications require the formulation of mixtures which possess low melting points, short pitch lengths and smectic transitions just below the required temperature-sensing region. Preferably the mixture or material should retain a low melting point and high smectic-cholesteric transition temperatures.
In general, thermochromic liquid crystal devices have a thin film of cholesterogen sandwiched between a transparent supporting substrate and a black absorbing layer. One of the fabrication methods involves producing an `ink` with the liquid crystal by encapsulating it in a polymer and using printing technologies to apply it to the supporting substrate. Methods of manufacturing the inks include gelatin microencapsulation. U.S. Pat. No. 3,585,318 and polymer dispersion, U.S. Pat. Nos. 1,161,039 and 3,872,050. One of the ways for preparing well-aligned thin-film structures of cholesteric liquid crystals involves laminating the liquid crystal between two embossed plastic sheets. This technique is described in UK patent 2,143,323.
One class of liquid crystal materials are known as polymer liquid crystals.
The unit that is the basic building block of a polymer is called a monomer.
In liquid crystal polymers the monomers can be attached together in essentially two ways. The liquid crystal part or mesogenic unit of the polymer may be part of the polymer backbone resulting in a main chain liquid crystal (MCLC) polymer. Alternatively, the mesogenic unit may be attached to the polymer backbone as a pendant group i.e. extending away from the polymer backbone; this results in a side chain liquid crystal (SCLC) polymer. These different types of polymer liquid crystal are represented schematically below. The mesogenic units are depicted by the rectangles. ##STR3##
The side chain liquid crystal polymer can generally be thought of as containing a polymer backbone with rigid segments (the mesogenic unit) attached along its length by a flexible.(or rigid) unit as depicted in the schematic representation on the following page. It is the anisotropic, rigid section of the mesogenic units that display orientational order in the liquid crystal phases. In order to affect the phases exhibited by the liquid crystal and the subsequent optical properties there are many features which can be altered, some of these features are particularly pertinent to side chain liquid crystal polymers. One of these features is the flexible part that joins the mesogenic unit to the polymer backbone which is generally referred to as a spacer group; the length and flexibility of this spacer group can be altered. ##STR4##
A number of side chain liquid crystal polymers are known, for example see GB 2146787 A.
Liquid crystal polyacrylates are a known class of liquid crystal polymer (LCP). LCPs are known and used in electro-optic applications, for example in pyroelectric devices, non-linear optical devices and optical storage devices. For example see GB 2146787 and Makromol. Chem. (1985) 186 2639-47.
Side chain liquid crystal polyacrylates are described in Polymer Communications (1988), 24, 364-365 e.g. of formula: ##STR5## where (CH.sub.2).sub.m --X is the side chain mesogenic unit and R is hydrogen or alkyl.
Side chain liquid crystal polychloroacrylates are described in Makromol. Chem. Rapid Commun. (1984), 5, 393-398 e.g. of formula: ##STR6##
A method for the preparation of homo- or co-polyacrylates having the following repeat unit is described in UK patent application GB 9203730.8 ##STR7##
R.sub.1 and R.sub.2 are independently alkyl or hydrogen. R.sub.3 is alkyl, hydrogen or chlorine, m is 0 or an integer 1-20, W is a linkage group COO or OOC, O and X is a mesogenic group.
The technique for aligning low molar mass liquid crystals is typically as follows. Transparent electrodes are fabricated on the surfaces of the substrates, the substrates typically being made of glass eg glass slides. In twisted nematic or super twisted nematic devices, for example, an alignment process is necessary for both substrates. A thin alignment layer is deposited to align the liquid crystal molecules, typically either organic or inorganic aligning layers are used, for example SiO deposited by evaporation is a typical inorganic alignment layer. One method to form the alignment layer involves rubbing the surface by textures or cloths. Polyimides have also been employed for the surface alignment of layers. Polyimide is coated onto the substrates bearing electrodes by a spinner and then cured to form a layer of approximately 50 nm thickness. Then each layer surface is repeatedly rubbed in substantially one direction with an appropriate material. If the liquid crystal molecules are deposited on this layer they are automatically aligned in the direction made by the rubbing. It is often preferable if the molecules possess a small angle pre-tilt typically 2-3.degree.. Higher pre-tilts are sometimes required.
The two substrates are then fixed together for example by adhesive and are kept separated by spacing materials. This results in uniform and accurate cell spacing. A typical adhesive is an epoxy resin. This sealing material is usually then precured. The electrodes may then be precisely aligned for example to form display pixels. The cell is then cured at, for example 100-150.degree. C. At this point the empty liquid crystal cell is complete.
It is at this point that the cell is filled with liquid crystal material. The opening size in the sealing area of the liquid crystal cell is rather small therefore the cell can be evacuated, for example in a vacuum chamber, and the liquid crystal forced into the cell via gas pressure. More than one hole in the sealing area may be used. The empty cell is put into a vacuum chamber and then the vacuum chamber is pumped down. After the cell has been evacuated the open region of the sealant is dipped into the liquid crystal material and the vacuum chamber is brought back to normal pressure. Liquid crystal material is drawn into the cell as a result of capillary action, external gases can be applied to increase the pressure. When the filling process is complete the hole or holes in the sealant is/are capped and the cell is cured at a temperature above the liquid crystal material clearing point to make the liquid crystal molecular alignment stable and harden the capping material.
Polymer liquid crystal molecules tend to be more viscous than low molecular weight liquid crystal materials and are therefore more difficult to align and more difficult to fill into devices. Only liquid crystal polymers with low molecular weights can be flow filled into a cell, and once a degree of polymerisation greater than around 30 or 40 repeat units is reached, most liquid crystal polymers become so viscous that flow filling cells is extremely difficult. Much slower cooling is needed in order to try and align liquid crystal polymers and this usually results in poor uniformity of alignment.
Poorly aligned liquid crystal molecules do not result in the fast switching high contrast devices that are generally required.
The above techniques are suitable for many liquid crystal materials including those devices which use liquid crystal materials which exhibit and utilise the smectic mesophase, eg ferroelectrics. Suitable alignment techniques may also be found in GB 2210469 B.
Ferroelectric LCDs by Dijon in Liquid Crystals Applications and Uses, vol 1 Ed. Bahadur, World Scientific Publishing Co. Pte. Ltd. 1990 pp 305-360 and references therein discusses alignment processes for smectic phases for low molar mass materials. The filling of cells is believed to be possible only in the isotropic or nematic phase due to the viscosity of smectic phases. Generally materials with the following phase sequence give good alignment: EQU I.fwdarw.N*.fwdarw.S.sub.A .fwdarw.S.sub.C * or I.fwdarw.S.sub.A .fwdarw.S.sub.C *
whereas materials with the following phase sequences are more difficult to align: EQU I.fwdarw.N*.fwdarw.S.sub.C *
Typically, therefore, in order to use a liquid crystal material in the smectic phase it will involve heating the material to the nematic or isotropic phase and allowing it to cool slowly into an aligned smectic state. Should this technique be applied to a polymer liquid crystal material then the cooling time is usually very much longer in order to assist the alignment, though very often the alignment is poor.
Materials and Assembling Process of LCDs by Morozumi in Liquid Crystals Applications and Uses. vol 1 Ed. Bahadur, World Scientific Publishing Co. Pte. Ltd. 1990pp 171-194 and references therein as the title suggests discusses methods for assembling liquid crystal devices.
Another method for aligning liquid crystal polymers may be found in UK patent application GB 9420632.3.
Another class of compounds which may be described as liquid crystal polymers are side chain liquid crystal cyclic polysiloxanes. These are described in the following references:
K. D. Gresham et al, J. Pol. Sci., Part A, Polymer chem., 32, 2039, 1994; PA0 I. Sledzinska et al. J. of inorganic and organometallic polymers, 4, 199, 1994; PA0 Kreuzer et al, Mol. Cryst. Liq. Cryst., 199, 345, 1991; PA0 Richards et al, J. chem. Soc. Commun., 2, 95, 1990; PA0 P. Spes et al, German Patent Application DE 3940148; PA0 Kreuzer et al U.S. Pat. No. 4.410.570.
Polysiloxanes may be represented by the following general formula: ##STR8## wherein n may range from 3 upwards, though more usually from 4 upwards. The positions of the substituents X and Y relative to each other allow for the existence of isomers. For example suppose that in the above formula n=4: ##STR9##
At each silicon one of the X and Y substituents ##STR10## will lie above the plane and the other will lie below the plane of silicons. The following notation may be used. ##STR11##
Thus when n=4, there are four possible isomers.
It is known to make polysiloxanes containing mesogenic side chains in either the X or Y positions. It is an object of the present invention to provide side chain liquid crystal cyclic polysiloxanes that exhibit enhanced properties over those already known.
Further it is the inventors' belief that no attempt has been made to synthesise pure isomers of side chain liquid crystal cyclic polysiloxanes. A pure isomer may exhibit different properties when compared with an isomeric mixture comprising that pure isomer.